1. Field of the Invention
The invention relates to management of power from two power sources in a hybrid electric vehicle powertrain.
2. Background Discussion
A series-parallel hybrid electric vehicle powertrain has two power sources for delivering driving power to vehicle traction wheels. An example of a series-parallel hybrid electric vehicle powertrain may be seen by referring to co-pending U.S. patent application Ser. No. 10/248,886, filed Feb. 27, 2003, entitled “Closed-Loop Power Control System for Hybrid Electric Vehicles,” now U.S. Pat. No. 6,991,053, dated Jan. 31, 2006, which is assigned to the assignee of the present invention.
The first power source in a powertrain of the kind disclosed in the co-pending patent application identified above comprises a combination of an engine and a generator that are mechanically coupled by a planetary gearset. A second power source is an electric drive system comprising a motor, the generator and a high voltage battery that are electrically coupled. The battery acts as an energy storing medium for the generator and the motor.
When the powertrain is operating in a driving mode that includes the first power source, the planetary gearset, together with the engine and the generator, cooperate to effect a power delivery characteristic that is analogous to the characteristic of a conventional continuously variable transmission in a vehicle powertrain. This is done by controlling generator speed, the generator being connected to the sun gear of the planetary gearset and the engine being connected to a planetary carrier. The ring gear of the planetary gearset is connected to the vehicle traction wheels through torque transfer gearing and a differential-and-axle-assembly.
Because of the fixed ratio of the planetary gearset and the variable generator speed, which achieve a decoupling of engine speed and vehicle speed, the planetary gearset acts as a power divider that divides engine output power and distributes power to torque transfer gearing and to the generator through separate power flow paths. The portion of the power delivered from the engine to the generator can be transmitted to the electric motor and then to the differential-and-axle assembly through the torque transfer gearing. Generator torque functions as a torque reaction as engine power is delivered through the planetary gearset.
When the vehicle is operating using the second power source, the electric motor draws power from the battery and provides driving torque to the vehicle traction wheels independently of the first power source. The two power sources can provide traction power for the vehicle either simultaneously or independently.
The two power sources must be integrated to work together seamlessly to meet a driver's demand for power within system power constraints while optimizing total powertrain system efficiency and performance. This requires a coordination of control of the two power sources.
As in the case of conventional continuously variable vehicle transmissions, a series-parallel hybrid powertrain can achieve better fuel economy and reduce undesirable exhaust gas emissions by operating the engine in its most efficient speed and torque operating region whenever possible. It also can potentially further improve fuel economy and quality of exhaust gas emissions, compared to a conventional continuously variable transmission, because the engine size can be reduced while providing the same vehicle performance. Still further, the engine can be stopped if operational conditions are not favorable for fuel economy and emissions purposes.
A vehicle system controller is used in a series-parallel hybrid powertrain to determine an engine torque and engine speed operating region to meet a driver demand for power while maintaining optimal fuel economy and optimum emissions quality under various vehicle operating conditions.
An example of a prior art hybrid powertrain system having a hydraulic pump and motor, rather than an electric generator and motor, is disclosed in U.S. Pat. No. 5,495,912. The combination of the hydraulic pump and motor, together with an accumulator, in the powertrain of the '912 patent, is analogous to the combination of an electric motor and a battery in a hybrid electric powertrain with divided power flow paths.
Both the powertrain system of the '912 patent and the powertrain system disclosed in the co-pending patent application identified above can provide an effective power command that is satisfied by two power sources, and both powertrain systems are capable of regenerative braking. In the case of the powertrain system of the '912 patent, the strategy used by the powertrain controller may achieve the best brake specific fuel consumption operating region for the engine for any given driver power demand by using the hydraulic pump and motor to control the engine. That is not sufficient, however, to achieve optimum total powertrain system efficiency since power losses in other components and sub-systems of the powertrain of the '912 patent are not taken into consideration.